How is it we can identify planets billions of light years away but not one in our backyard?
We look at a star, and measure a decrease in brightness that corresponds to a planet passing in front of it.
We can also look for light reflecting off a large planet near its star.
Problem with Planet Y is it’s not positioned for us to observe that way, reflects too little light, and we don’t even know where to look.
Too far out to reflect enough sunlight to be detected.